Many different methods for the preparation of epoxides have been developed. One such method involves the epoxidation of an olefin in a liquid phase reaction using an organic hydroperoxide as the oxidizing agent and certain solubilized transition metal compounds as catalyst. The early work in the field concluded that optimum epoxidation rates and selectivity to epoxide generally are obtained using metallic catalysts which are soluble in an organic reaction medium.
A distinct disadvantage of an epoxidation process which utilizes a soluble metallic compound as catalyst is the difficulty associated with recovering the catalyst for reuse in subsequent runs. When the other components of an epoxidation reaction mixture (typically, epoxide, unreacted olefin, solvent, unreacted hydroperoxide, and the alcohol derived from the reacted hydroperoxide) are relatively volatile, these components may be separated from the soluble non-volatile catalyst by distillation and the catalyst recovered in the form of a bottoms stream. A problem associated with such a method, however, is that the bottoms stream may tend to accumulate certain heavy substances such as acids and polymers which may have a deleterious effect on epoxide selectivity or olefin conversion when the stream is reused. The catalyst may also have a tendency to precipitate from solution if the bottoms stream is overly concentrated; recycle of a relatively large bottoms stream may thus be required, which will detrimentally affect the productivity of the epoxidation process. It would therefore be highly desirable to develop an insoluble (heterogeneous) epoxidation catalyst which has high activity and selectivity and which may be readily recovered in active form from an epoxidation reaction mixture by filtration or similar separation techniques or which may be utilized in the form of a fixed bed or the like.
U.S. Pat. No. 4,367,342 discloses an olefin epoxidation process wherein an olefin is contacted with an organic hydroperoxide in the presence of an insoluble catalyst comprised of an inorganic oxygen compound of titanium. Such catalysts are further described in British Pat. No. 1,332,527 and U.S. Pat. Nos. 4,021,454, 3,829,392 and 3,923,843. Unfortunately, catalysts prepared in accordance with the procedures described in these references have less than optimum activity and selectivity. Incorporation of relatively high levels of titanium into catalysts of this type, in an attempt to improve catalyst activity, has also been challenging.
Consequently, it would be highly desirable to develop alternative methods of synthesizing heterogeneous titanium-containing catalysts which avoid the shortcomings of prior art procedures and reliably and conveniently provide materials having higher activity and selectivity in olefin epoxidation reactions.
British Patent No. 1,332,527 teaches a process for preparing an improved silica-titania catalyst characterized by impregnating an inorganic siliceous solid with a substantially non-aqueous solution of a titanium compound in an oxygen-substituted hydrocarbon solvent, removing solvent from the impregnated siliceous solid, and thereafter calcining the impregnated siliceous solid. Suitable solvents for this purpose are limited to oxa and/or oxo-substituted hydrocarbons which are liquid at ambient conditions and comprise generally from 1 to 12 carbon atoms. Such solvents include alcohols, ketones, ethers and esters. According to the patent, the reason why silica-titania catalyst produced by a process where an oxygen-substituted hydrocarbon impregnation solvent is used has improved properties compared to similar catalysts prepared by other methods is that such catalyst has a more uniform, non-agglomerated content of titanium dioxide.
A later-filed patent application (EP 345,856) discloses the preparation of epoxidation catalysts which are alleged to be more active than the analogous catalysts obtained by previously known procedures. EP 345,856 teaches impregnation of silica with a gaseous stream of titanium tetrachloride, followed by calcination, hydrolysis, and, optionally, silylation. In a comparative example, a catalyst prepared by silica impregnation with a solution of tetra isopropyl ortho-titanate, complexed with acetyl acetone in isopropanol as solvent, was found to be 4.5 times less active than the catalyst prepared by vapor phase impregnation with titanium tetrachloride. The implication of this disclosure is that it is not possible to attain similar catalytic activity, while maintaining high epoxide selectivity, using a liquid phase rather than vapor phase impregnation process.
EP 734,764 teaches an improvement to the liquid phase impregnation process disclosed in British Pat. No. 1,323,527 wherein after impregnating silica with a solution of a titanium compound in an oxygen-containing organic solvent and removing the impregnation solvent the catalyst is washed with a washing solvent and then calcined. Preferably, the washing solvent is an alcohol. Washing prior to calcination is taught to be necessary in order to obtain a catalyst which is excellent both in activity and selectivity although examination of the comparative examples included in EP 734,764 indicates that only very modest improvement in catalyst performance is actually achieved using this procedure. Another practical disadvantage of this procedure is that large volumes of waste solvent are generated which must be either disposed of or recycled after purification. Such disposal or purification will substantially increase the cost of producing the catalyst. Another disadvantage is that it is difficult to achieve high levels of titanium incorporation, since washing tends to remove substantial amounts of titanium with this effect being even more pronounced using large quantities of the titanium reagent relative to the silica. Moreover, this procedure does not permit precise control of the final titanium content of the catalyst.
We have now discovered an effective, convenient method of producing catalyst compositions having epoxidation activity and selectivity at least comparable to catalysts obtained by the techniques taught in EP 345,856.